Polymer compositions useful for molding applications, specifically the manufacture of caps and closures for bottles are well known. Screw closures for example, are typically made from polypropylene (PP) in order to achieve the necessary cap strength, however, an inner liner composed of a soft polymer is required to provide necessary seal properties. Also a PP closure alone does not have good organoleptic properties, which is another reason to use a liner. The soft inner liner can be made from ethylene/vinyl acetate (EVA), polyvinyl chloride (PVC), butyl rubber or other suitable material. The two-part cap is costly, and single part constructions may reduce costs.
One-piece closures, such as screw caps, have more recently been made from polyethylene (PE) resins. To date the use of high density resin is required if the closures are to have sufficient stiffness, while good flow properties combined with an ability to resist cracking over time (as measured by for example, environmental stress crack resistance (ESCR)) is also desirable. Further, closures should be rapidly produced while not suffering from anisotropic shrinkage when released from a mold. Such closures, for example, screw cap closures for water and other drink bottles, should, even when produced in high turnover, consistently satisfy strict dimensional tolerances to provide a consistent fit and seal and to maintain product integrity. Polyethylene closures alone have better organoleptic properties than PP closures alone.
Polyethylene blends produced with conventional Ziegler-Natta or Phillips type catalysts systems can be made having suitably high density and ESCR properties, see for example, WO 00/71615 and U.S. Pat. No. 5,981,664. However, the use of conventional catalyst systems may produce significant amounts of low molecular weight polymer chains having high comonomer contents, which results in resins having non-ideal organoleptic properties.
Examples of high density multimodal polyethylene blends made using conventional catalyst systems for the manufacture of caps or closures are taught in U.S. Pat. Nos. 7,750,083; 7,750,082; 7,790,826; 8,044,160; and 8,129,472; U.S. Patent Applications 2007/0213468 and 2008/0287608, as well as, WO 2007/060007 and EP 2,017,302A1. Further high density, multimodal polyethylene blends made by employing conventional Ziegler-Natta catalysts are disclosed in U.S. Patent Applications 2009/0198018; 2009/0203848; 2008/0221273; 2010/0084363 and 2012/0022214.
In contrast to traditional catalysts, the use of so called single site catalysts (such as “metallocene” and “constrained geometry” catalysts) provides resin having lower catalyst residues and improved organoleptic properties as taught by U.S. Pat. No. 6,806,338. The disclosed resins are suitable for use in molded articles. Further resins comprising metallocene catalyzed components which are useful for molding applications are described in U.S. Pat. Nos. 7,022,770; 7,307,126; 7,396,878; 7,396,881; and 7,700,708.
A monomodal polyethylene composition which is useful in the preparation of a screw closure has been made using a mixed metallocene catalyst system as described in U.S. Pat. No. 8,039,569.
U.S. Patent Application 2011/0165357A1 discloses a blend of metallocene catalyzed resins which is suitable for use in pressure resistant pipe applications.
U.S. Patent Application 2006/0241256A1 teaches blends formulated from polyethylenes made using a hafnocene catalyst in the slurry phase.
A bimodal resin having a relatively narrow molecular weight distribution and long chain branching is described in U.S. Pat. No. 7,868,106. The resin is made using a bis-indenyl type metallocene catalyst in a dual slurry loop polymerization process and can be used to manufacture caps and closures.
U.S. Pat. No. 6,642,313 discloses multimodal polyethylene resins which are suitable for use in the manufacture of pipes. A dual reactor solution polymerization process is used to prepare the resins in the presence of a phosphinimine catalyst.
Narrow molecular weight distribution polyethylene blends comprising a metallocene produced polyethylene component and a Zielger-Natta or metallocene produced polyethylene component are reported in U.S. Pat. No. 7,250,474. The blends can be used in blow molding and injection molding applications such as for example, milk bottles and bottle caps respectively.
For further references describing relevant polyethylene compositions see: U.S. Pat. Nos. 7,875,690; 6,545,093; 8,129,489; 6,063,871; 5,382,630; 5,382,631; 7,928,051; 6,809,154; 7,592,395; 6,194,520; 5,858,491; 6,946,521; and 5,494,965 as well as U.S. Patent Application 2010/0121006 and 2011/0136983.
In U.S. Pat. No. 8,022,143 we disclosed a resin composition having a good balance of toughness, ESCR, processability, and organoleptic properties for use in the manufacture of caps and closures. The resins were made using a single site catalyst system in a dual reactor solution polymerization process, to provide bimodal polyethylene compositions in which comonomer was present in both a high and a low molecular weight component. The disclosed resins had a normal comonomer distribution in that the low molecular weight component had a larger amount of comonomer than did the high molecular weight component.
In CA Patent Application 2,752,407 we disclosed resins having improved ESCR, good organoleptic properties, balanced rheological and mechanical properties and which were suitable for use in the manufacture of molded articles such as closures for bottles.